1. Field of the Invention
The present invention relates to a light modulator such as a display device and an optical switch that employ liquid crystals.
2. Description of the Prior Art
Thanks to the technical development, the TN-type TFT liquid crystal displays offer high displaying quality. A color reproduction range has been improved to such a level as to be near to that of CRT, and the contrast ratio comparable to or better than that of CRT has also been achieved. The problem of viewing angles of liquid crystals has been solved by the invention of the inplane switching system or the vertical alignment system, resulting that display devices with wide viewing angles and high image quality have come into existence.
However, there still exist some problems. Since these display devices use backlight, they consume a lot of power and in bright places such as the outdoors, images on the screen is difficult to see because of ambient light brighter than the brightness of the screen.
For these reasons, reflective display devices, which make use of the ambient light for illumination, are in the limelight. Since the reflective display devices consume less power and provide clear images in brighter places, they are suitable for display devices of mobile computers and portable phone terminals which are used outsides.
As reflective display devices, liquid crystal display devices using TN system and ECB system are known. However, since these use polarizer films which absorb more than one-half of outside light is absorbed, there are the problems of a low refrectance and a difficulty in displaying white.
On the other hand, as a system not using polarizer films, the PCGH (phase change guest host) system is known. According to the PCGH system, with dichroic dyes added to cholesteric liquid crystals, when no electric field is applied, specific colors are displayed by aligning liquid crystal molecules and the dichroic dyes in a direction parallel to the substrates due to a planer state of the cholesteric liquid crystals; when an electric field is applied, cholesteric liquid crystal layers are made transparent by arranging the liquid crystal molecules and the dichroic dyes in a direction perpendicular to the substrates. However, the PCGH has a problem that a high contrast cannot be obtained because it is difficult to obtain a dye with a high dichroism ratio.
Accordingly, as reflective liquid crystal display devices without polarizer films, display devices using selective reflection of cholesteric liquid crystals are in the limelight. Generally, cholesteric liquid crystals can be obtained by a adding a chiral agent to the nematic liquid crystals are used. By adding the chiral agent, the cholesteric liquid crystals form a cyclic helical structure, cause a cyclic change of a refractive index, and effectively reflect light of specific wavelengths by Bragg reflection. This state is called a planer state, and by changing the amount of the chiral agent to be added, a wavelength for selective reflection can be freely changed to display blue, green, or red.
By injecting a cholesteric liquid crystal between a pair of transparent substrates each having a transparent electrode formed thereon, the planer state is obtained and the cell appears colored, reflecting light of specific wavelengths. The planer state is one of stable states of the cholesteric liquid crystal and can last even if no electric field is applied.
When a sufficiently high voltage is applied between the upper and lower electrodes in the state, the helical structure of the cholesteric liquid crystal disappears, and if the cholesteric liquid crystal has positive dielectric anisotropy, the liquid crystal molecules are arranged in a direction perpendicular to the substrate, that is, the direction of an electric field, and a cyclic change of refractive indexes disappears. This state is called a homeotropic state. In this case, selective reflection does not occur and the cholesteric liquid crystal goes into a transparent state in which all incident light is transmitted. Therefore, black color is appeared by forming a light absorption layer on the back of a substrate opposite to the outside light incidence side.
When an intermediate voltage is applied between the upper and lower electrodes in the state, the cholesteric liquid crystal goes into a state in which the helical axis is parallel to the substrate. This state is called a focal conic state. In this case, selective reflection does not occur, and the cholesteric liquid crystal goes into a state in which incident light is reflected only a little, or if the cell gap is small, into a transparent state in which all incident light is transmitted. The focal conic state is the another stable state of the cholesteric liquid crystal and can last even if no electric field is applied.
However, cholesteric liquid crystals change slowly in state; for example, transition from the homeotropic state to the planer state requires about one second. Accordingly, the cholesteric liquid crystals are unsuitable for display devices for which fast rewriting is required, such as when displaying moving images.
On the other hand, as a sort of liquid crystals, ferroelectric liquid crystals or ferroelectric smectic-C* liquid crystals are available, and as display devices using them, SSFLC (Surface Stabilized Ferroelectric Liquid Crystal) display devices as described in Japanese Published Unexamined Patent Application No. Hei 5-241527 and a literature xe2x80x9cSID 94 DIGEST p845-847xe2x80x9d are known.
The SSFLC display devices, which have a ferroelectric liquid crystal sandwiched between a pair of transparent electrodes and have two polarizer films orthogonal to each other disposed at both ends of the ferroelectric liquid crystal, can change light permeability and switch between a light transmission state and a shading state by inverting voltages applied between the transparent electrodes.
As other devices using ferroelectric liquid crystals, optical switch devices such as DHF (helical structure variable distortion type) and TMS (transient light scattering type) are known.
Ferroelectric liquid crystals, which have spontaneous polarization, can fast switch the orientation of the molecules.
However, the above-described SSFLC display devices, which use backlight and polarizer films to display images, have the problem that, although they enable fast switching between a light transmission state and a shading state, like transmission liquid crystal display devices such as conventional TN-type TFT liquid crystal displays, they have poor images in bright places and consume much electric power. This is also true for DHF and TMS optical switch devices.
Accordingly, the present invention provides a light modulator that can provide bright and contrasty reflective displays or optical switchings without using polarizer films, and enables fast display switching and optical switching.
A light modulator of the present invention has: a light modulation element and a driving circuit that drives it, wherein the light modulation element has a ferroelectric liquid crystal filled between a pair of substrates at least one of which is transparent, and is provided with a pair of electrodes for applying electric fields to the ferroelectric liquid crystal, the ferroelectric liquid crystal selectively reflecting light of specific wavelengths in a visible region due to a planer state in which helical axes are perpendicular to or almost perpendicular to the substrates, and wherein the driving circuit drives the light modulation element, at least, by a driving mode of changing the ferroelectric liquid crystal to a selective reflection state due to the planer state and a transparent state due to a homeotropic state in which a helical structure disappears.
The driving circuit may drive the light modulation element by the driving mode and another driving mode of selectively changing the ferroelectric liquid crystal to the selective reflection state due to the planer state and the transparent state due to a focal conic state in which the helical axes are parallel to or almost parallel to the substrates.
A light modulator of the present invention is configured as described above, taking advantage of the selective reflection of ferroelectric liquid crystals. According to the light modulator of the present invention, a bright, and contrasty reflective display or optical switching can be performed without using polarizer films, while display switching and optical switching can be performed fast, so that the display device can display moving images.